I only use Efitec 98 from Repsol (reliable quality!) at around €1.30 EUR per litre, which works out about 1.15 GBP/l. I think that's reasonable and like Mr.Singh, I find using 98RON does seem to give a little bit more power.
Also, my own reading tells me that higher RON suits higher compression engines, and I think (relatively) high compression is one way Mitsubishi squeezes HP out of the 1.1 3Cyl engine, so it seems logical to me too, to stick with 98 RON.
Can't say I ever tanked the car fully as I don't use it for long journeys etc.. most I ever put in was probably 25l. It's just a lot for me to throw 50-odd euro into the tank in one go, though to be fair it'd probably last me for months.
How I wish for the return of those halcyon days where your litres counted up faster than the pounds/euros counted down...